In the field of semiconductor technology, the performance of semiconductor devices are continuously pushed to higher limits. However, for a specific type of semiconductor device, a tradeoff in the design of the semiconductor device may sometimes be required such that a characteristic of the semiconductor device is optimized instead of another. Such a tradeoff may arise because of the final application intended for the semiconductor device, wherein a characteristic may be preferred as compared to another, or may also arise because of limitations in the manufacturing of the semiconductor device.
Referring in particular to the example of a bipolar junction transistor (BJT) comprising a collector region, a base region and an emitter region, the critical characteristics representative of the performance of the BJT are the common emitter current gain, the specific on-resistance and the breakdown voltage. It is known that, for a specific doping concentration, the base region of the BJT is preferably as thin as possible in order to obtain a high current gain. However, the minimum thickness of the base region is limited by the so-called punch-through effect wherein, for a too thin base region, the depletion region at the base-collector boundary may meet the depletion region at the base-emitter boundary. The depletion region is a space charge region of the target substrate in which the semiconductor material is depleted of charge carriers. Under such conditions, the collector region is effectively short-circuited to the emitter region, and a large current flows between the collector region and the emitter region. Thus, for a specific doping concentration, the thickness of the base region is, on the one hand, preferably increased for increasing the electric breakdown voltage of the BJT and, on the other hand, preferably decreased for increasing the gain of the BJT.
Further, the above mentioned punch-through effect, wherein the BJT has virtually no base (or wherein the effective base width is operatively reduced to zero), may occur more easily (i.e. at a lower collector-base voltage) if there are defects at the boundary between the emitter region and the base region.
In the fabrication of a mesa etched BJT, or even other kinds of semiconductor devices comprising a mesa structure or the like, a critical processing step is plasma etching (or dry etching) which is used for forming a structure having vertical or near vertical side wall in a semiconductor material, e.g. silicon carbide (SiC). Dry etching is normally used to form the emitter region and to terminate the base-collector junction. However, dry etching at a vertical or near vertical side wall in a semiconductor material such as e.g. SiC may result in the formation of a trench at the base of the wall, which may deteriorate the functioning of the resulting semiconductor device. In the following, the formation of such a trench will be referred to as the “trench effect”.
For a BJT, the formation of such a trench at the boundary between the base region and the emitter region may result in that the effective thickness of the base region is reduced and does not correspond to the initially intended thickness of the base region deposited or grown on the substrate, thereby resulting in that the distance for punch-through of a high electric field to the top surface (the emitter region) decreases. Thus, for reducing the punch through effect, the base region of the BJT has to be sufficiently thick, which, however, reduces the gain of the BJT. In addition, during subsequent high temperature annealing of the semiconductor device, the material close to the top surface may redistribute and may be transported to the trench. The material transport in combination with the trench may cause an uncontrolled doping and poor material quality in a sensitive part of the transistor (e.g. at the emitter/base junction), thereby degrading the performance of the BJT.
Thus, there is a need for providing new types of structure for semiconductor devices and/or new manufacturing methods that would alleviate at least some of the above-mentioned drawbacks and/or enable to provide semiconductor devices with improved characteristics.